DC-DC converter

ABSTRACT

A DC-DC converter includes an Nch FET 1  which is series connected between an input terminal and an output terminal, and an Nch FET 2  which is connected between the output terminal side of the Nch FET 1  and a ground terminal. On the output terminal side of the circuits, a smoothing circuit and a comparator circuit are connected. The output side of the comparator circuit is connected to an on-time limiter circuit, and the on-time limiter circuit is connected to an H/S driver circuit for controlling the Nch FET 1  through an inverter and directly connected to an L/S driver circuit for controlling the Nch FET 2 . Here, when a switching control signal for turning on the Nch FET 1  is input to the on-time limiter circuit from the comparator, the on-time limiter circuit detects a time in the on state and outputs a signal by which the Nch FET 1  is temporarily off controlled.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates a DC-DC converter for step-down converting an input DC voltage to a DC voltage having a predetermined value, and more particularly, to a step-down ripple detection type self-oscillation converter.

2. Description of the Related Art

At the present time, low-voltage and high-current DC-DC converters are required because of the demand for computer power-supply circuits. As representatives of such DC-DC converters, PWM control converters and ripple detection type self-oscillation converters (hereinafter, simply referred to as ripple converters) are utilized. Among them, ripple converters have lately attracted some attention even though the later-developed PWM control converters have been overwhelmingly popular, because the response to load variations is good with ripple converters.

FIG. 5 shows a basic circuit of a ripple converter.

As shown in FIG. 5, in the ripple converter, a PNP transistor Tr₁ defining a switching element and an inductor L₀₁ which are series connected, are disposed between an input terminal 3 at which an input voltage Vin is input and an output terminal 4 at which an output voltage Vout is output, and a flywheel diode connected between the connection point of the PNP transistor Tr₁ to the inductor L₀₁ and the ground is provided. Furthermore, a comparator 10, in which a voltage in accordance with an output voltage Vout is input to a non-inverting input terminal, a reference voltage Vo is input to an inverting input terminal, and a switching control signal is output to the PNP transistor Tr₁, is provided.

In such a ripple converter, while the PNP transistor Tr₁ is in the off state, when the output voltage Vout becomes less than the reference voltage Vo, a low signal output from the comparator is input to the base of the PNP transistor Tr₁ and the PNP transistor Tr₁ is turned on. Then, the output voltage Vout increases such that the PNP transistor Tr₁ is turned on. On the other hand, while the PNP transistor Tr₁ is in the on state, when the output voltage Vout becomes greater than the reference voltage Vo, a high signal is output from the comparator and input to the base of the PNP transistor Tr₁, and as a result, the PNP transistor Tr₁ is turned off. Then, the output voltage Vout is reduced such that the PNP transistor Tr₁ is turned off. By repeating such a control operation, the output voltage Vout goes up and down around a voltage that is close to the reference voltage Vo and an output voltage Vout which substantially equal to the reference voltage Vo is obtained.

In a practical circuit of such a ripple converter disclosed in Japanese Unexamined Patent Application Publication No. 9-51672, a ripple converter having a structure in which a switching element of a Pch FET and a choke coil are connected between an input terminal and an output terminal, and a diode is connected between the connection point of the Pch FET to the choke coil and the ground potential. Furthermore, in the ripple converter described in JP 9-51672, a comparator in which a voltage in accordance with an output voltage is input to an inverting input terminal and a reference voltage is input to a non-inverting input terminal and a drive IC which outputs a switching control signal to the Pch FET in accordance with the output voltage of the comparator are provided. Then, in this ripple converter, a desired output voltage is obtained from input voltage such that the output voltage is compared to a reference voltage and, based on the results of the comparison, the Pch FET is switched.

When compared with Nch FETs, Pch FETs have less desirable characteristics. So if the Pch FET is used in a synchronous rectification type ripple converter, the conversion efficiency is degraded. Therefore, a synchronous rectification type ripple converter using the Nch FET is required, instead of the Pch FET.

In such a ripple converter using an Nch FET, a higher voltage than the source voltage must be applied to the gate in order to drive the FET, and a bootstrap circuit shown in FIG. 6A is typically required on the control signal input side of the Nch FET.

FIG. 6A is a circuit diagram of a synchronous rectification type ripple converter using an Nch FET in which a related bootstrap circuit is provided, and FIG. 6B shows voltage waveforms at fixed points (point A and point B) in the circuit shown in FIG. 6A.

As shown in FIG. 6A, an Nch FET₁ (hereinafter, referred to as an FET₁) and an inductor L₁ are series-connected between an input terminal 3 at which an input voltage Vin is input and an output terminal 4 at which an output voltage Vout is output. A bootstrap circuit including a bootstrap diode D₀ and a bootstrap capacitor C₀ which are series connected to each other is provided between the drain and source of the FET₁. An H/S driver circuit 1 is connected to the gate of the FET₁. The voltages at both ends of the bootstrap capacitor C₀ are applied to the H/S driver circuit 1. The gate of an Nch FET₂ (hereinafter, referred to as an FET₂) is connected to an L/S driver circuit 2, the drain is connected to the connection point between the FET₁ and the inductor L₁, and the source is grounded. The L/S driver circuit 2 is connected between the input terminal 3 and the ground. Furthermore, a capacitor C₁ is connected between the connection point of the inductor L₁ to the output terminal 4 and the ground. Moreover, an illustration of a comparator which detects the ripple of an output voltage Vout and supplies a FET switching signal to the H/S driver circuit 1 and the L/S driver circuit 2 is omitted.

In such a ripple converter, the voltages having waveforms shown in FIG. 6B are generated at point B and point A in FIG. 6A, such that the output characteristics of the FET₁ and FET₂ are reversed and synchronized therebetween. When the FET₁ is turned on at point A (the FET₂ is turned off), substantially the same voltage as the input voltage Vin is supplied at point A, and, when the FET₁ is turned off (the FET₂ is turned on), approximately zero volts (ground potential) are supplied at point A. Then, a voltage obtained by subtracting the voltage drop in the bootstrap diode D₀ from the input voltage Vin is applied to the bootstrap capacitor C₀, so as to charge the capacitor C₀. Therefore, at point B, when the FET₁ is turned on (the FET₂ is turned off), a voltage increasing above the input voltage Vin by the electric charge charged in the bootstrap capacitor C₀ is generated, and, when the FET₁ is turned off (the FET₂ is turned on), a voltage obtained by subtracting the voltage drop in the bootstrap diode D₀ from the input voltage Vin is generated.

In such a structure, as described above, since an increased drive control voltage of the FET₁ is generated from the H/S driver circuit 1, such that the charged bootstrap capacitor C₀ is discharged, the FET₁ must be repeatedly turned on and off. That is, the FET₁ is not continuously in the on state and the FET₁ must be switched on and off at a certain duty factor. Moreover, a duty factor mentioned here represents the ratio, expressed by percentage, of the time that the FET₁ is in the on state all of the time, that is, if the FET₁ is constantly in the on state, the duty factor is 100%.

However, as shown in FIG. 5, the ripple converter does not include any circuit for controlling a duty factor and the duty factor cannot be effectively controlled in principle. Therefore, when the input voltage Vin is decreased, the output voltage Vout does not reach the reference voltage Vo, and the FET₁ is constantly in the on state, then, as a result, the bootstrap capacitor C₀ is completely discharged. When the bootstrap capacitor C₀ is discharged, the voltage across the capacitor C₀ cannot be increased, a voltage required for the gate of the FET₁ cannot be supplied, and as a result, the FET, is turned off. Therefore, the output voltage Vout is further reduced.

Thus, when the output voltage Vout is reduced, as described above, a voltage cannot be supplied from the bootstrap capacitor C₀. Accordingly, the FET₁ cannot be turned on, and the output voltage Vout is further reduced. That is, the FET₁ cannot be stably operated such that the bootstrap capacitor is completely discharged.

SUMMARY OF THE INVENTION

To overcome the problems described above, preferred embodiments of the present invention provide a ripple converter type DC-DC converter in which a predetermined voltage is stably output.

According to a preferred embodiment of the present invention, a DC-DC converter includes an Nch FET for on-off controlling an input voltage, a smoothing circuit for smoothing a switched output voltage from the Nch FET to output the smoothed voltage, and a comparator circuit for comparing a voltage output from the smoothing circuit with a set voltage to output a switching control signal for the on-off controlling. In the DC-DC converter, when a continuous on time of the Nch FET is detected and the continuous on time reaches a predetermined time, a switching control signal for off controlling the Nch FET is output to the Nch FET.

When the Nch FET is in the off state (the Nch FET is turned off) and the output voltage of the smoothing circuit (output voltage of the converter) is less than a set voltage, a switching control signal for turning on the Nch FET is output from the comparator circuit. Here, when the on state continues for a predetermined time, that is, the state where the voltage input to the comparator is less than a set voltage, continues for a predetermined time or more, the on-time limiter circuit outputs a switching control signal for temporarily turning off the Nch FET. Thus, the Nch FET is turned off and the bootstrap capacitor connected to the switching circuit for controlling the switching of the Nch FET is rapidly charged. Even if such an off control is performed, since a voltage in accordance with the DC output voltage is less than the reference voltage, a switching control signal for turning on the Nch FET once again is input to the Nch FET and the Nch FET is turned on. In this manner, the bootstrap circuit continuously functions, such that the switching of the Nch FET is repeated.

Furthermore, according to a DC-DC converter of a preferred embodiment of the present invention, in the on-time limiter circuit, a time limit in the on state is about three to about ten times as long as a cycle of the switching control signal during normal operation.

Thus, the off state is more effectively achieved, such that the time limit in the on state is set as described above.

According to preferred embodiments of the present invention, even if the Nch FET continues to be in the on state, since a temporary time in the off state is provided, the bootstrap capacitor can be charged during the off time. Therefore, the Nch FET is consecutively maintained in the on state, and the Nch FET is stably operated. Thus, a ripple converter type DC-DC converter for stably outputting a predetermined voltage is provided.

Furthermore, according to preferred embodiments of the present invention, a ripple converter type DC-DC converter for more stably outputting a predetermined voltage is provided, such that the time limit in the on state is set in an appropriate range as described above.

These and other features, elements, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments of the present invention with reference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram showing the structure of a ripple converter of a preferred embodiment of the present invention;

FIG. 2 is the detailed circuit diagram of an on-time limiter circuit shown in FIG. 1;

FIG. 3 shows waveforms of a load output current, a ripple current of an inductor, and a switching control signal;

FIG. 4 shows characteristics of a smoothing circuit;

FIG. 5 shows a basic circuit of a ripple converter;

FIG. 6A is a circuit diagram of a synchronous rectification type ripple converter using an Nch FET having a related bootstrap circuit contained therein; and

FIG. 6B shows waveforms at fixed points (point A and point B) in the circuit diagram in FIG. 6A.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

A ripple converter type DC-DC converter according to a preferred embodiment of the present invention is described with reference to FIGS. 1 to 4.

FIG. 1 is a circuit diagram showing the structure of a ripple converter of the present preferred embodiment.

Furthermore, FIG. 2 shows the detailed circuit of an on-time limiter circuit 11 in FIG. 1.

As shown in FIG. 1, in the ripple converter of the present preferred embodiment, an Nch FET₁ (hereinafter, simply referred to as an FET₁) and an inductor L₁ are series-connected in order from the side of an input terminal 3 between the input terminal 3 and an output terminal 4. Here, in the FET₁, the drain is connected to the input terminal 3 and the source is connected to the inductor L₁. Furthermore, the gate of the FET₁ is the control signal output terminal of an H/S driver circuit 1. Moreover, between the drain and source of the FET₁, a bootstrap circuit is provided which includes a bootstrap diode D₀ and a bootstrap capacitor C₀ which are series-connected. In the bootstrap circuit, the anode of the bootstrap diode D₀ is connected to the drain of the FET₁ and the input terminal 3, and the bootstrap capacitor C₀ is connected to the source of the FET₁.

In the driver circuit 1, the power-supply input terminal is connected to the connection point between the cathode of the bootstrap D₀ and the bootstrap capacitor C₀ in the bootstrap circuit, and the ground-side terminal is connected to the connection point between the bootstrap capacitor C₀ and the source of the FET₁. Thus, the voltage charged in the bootstrap capacitor C₀ of the bootstrap circuit is applied to the H/S driver circuit. Furthermore, an on-time limiter circuit 11, to be described later, is connected to a signal input terminal of the H/S driver circuit 1 through an inverter 12, and a signal output from the on-time limiter circuit 11 is inverted and input to the signal input terminal.

An Nch FET₂ (hereinafter, referred to as an FET₂) is connected between the connection point of the FET₁ to the inductor L₁ and the ground terminal 5. Here, in the FET₂, the drain is connected to the connection point between the FET₁ and the inductor L₁ and the source is connected to the ground terminal 5. Furthermore, the gate of the FET₂ is connected to the control signal output terminal of an L/S driver circuit 2.

In the L/S driver circuit 2, the power-supply input terminal is connected to the input terminal 3, and the ground-side terminal is connected to the ground terminal 5. Furthermore, to the signal input terminal of the L/S driver circuit 2, the on-time limiter circuit 11 is connected and a signal output from the on-time limiter circuit 11 is directly input.

Between the connection point of the inductor L₁ to the output terminal 4 and the ground terminal 5, a capacitor C₁ is connected and a series circuit of resistors R₁ and R₂ is connected in parallel with the capacitor C₁.

The connection point of the resistors R₁ and R₂ is connected to a non-inverting input terminal of a comparator 10.

The inverting input terminal of the comparator 10 is connected to the plus side of a reference voltage V₀ and the minus side-of the reference voltage is grounded. Furthermore, the positive-side power-supply terminal of the comparator 10 is connected to the input terminal 3, and the negative-side power-supply terminal is grounded. Moreover, a signal output terminal of the comparator 10 is connected to a signal input terminal of the on-time limiter circuit 11. A high signal and a low signal generated by comparing the voltage input to the inverting input terminal of the comparator 10 and the voltage input to the non-inverting input terminal correspond to a switching control signal of the present invention.

Furthermore, a smoothing circuit preferably includes the inductor L₁ and the capacitor C₁, and a comparator circuit 14 preferably includes the comparator 10, the resistors R₁ and R₂, and the reference power supply.

The on-time limiter circuit 11 has the signal input terminal, the power-supply terminal, and the signal output terminal. The power-supply terminal is connected to the input terminal 3, and the signal output terminal is connected to the L/S driver circuit 2 and to the H/S driver circuit 1 through the inverter 12.

A specific structure of the on-time limiter circuit 11 is described with reference to FIG. 2.

The input portion of a buffer circuit 20 is connected to the signal output terminal of the comparator 10 through a resistor R11 and to the input terminal through a resistor R12. The output portion of the buffer circuit 20 is connected to the H/S driver circuit 1 through the inverter 12 and directly to the L/S driver circuit 2. Furthermore, the output portion of the buffer circuit 20 is grounded through a series circuit of resistors R₁₃ and R₁₄, and the connection point of the resistors R₁₃ and R₁₄ is connected to the base of an NPN transistor Tr₁₁.

In the NPN transistor Tr₁₁, the emitter is grounded, and the collector is connected to the base of an NPN transistor Tr₁₂, through a resistor R₁₆. Furthermore, the collector of the NPN transistor Tr₁₁ is connected to the input terminal 3 through a resistor R₁₅ and grounded through a capacitor C₁₁. The resistor R₁₅ and the capacitor C₁₁ define a time-constant circuit for setting the maximum value of an on time.

In the NPN transistor Tr₁₂, the emitter is grounded and the collector is connected to the base of a PNP transistor Tr₁₃ through a resistor R₁₇.

In the PNP transistor Tr₁₃, the emitter is connected to the input terminal 3 and the collector is connected to the input portion of the buffer circuit 20.

The ripple converter having the above-described structure operates as described below.

At the initial state (where the output voltage is zero volt), a zero-volt voltage is input to the non-inverting input terminal of the comparator 10. Therefore, the comparator 10 outputs zero volt (a low signal). The on-time limiter circuit 11 outputs the low signal to the inverter 12 and the L/S driver circuit 2 through the buffer circuit 20, and the inverter 12 inverts the low signal and outputs a high signal of a predetermined voltage to the H/S driver circuit 1. At this time, the NPN transistor Tr₁₁ in the on-time limiter circuit 11 is turned off and the capacitor C₁₁ is charged by an input voltage V_(in) through the resistor R₁₅ at a predetermined time constant.

When the high signal is input, the H/S driver circuit 1 generates a gate-on control signal (hereinafter, referred to as an on control signal) and outputs the signal to the gate of the FET₁. Then, since the bootstrap capacitor C₀ is charged by the input voltage V_(in), an on control signal having a voltage that is greater than the input voltage V_(in) is input to the gate of the FET₁. On the other hand, when a low signal is input, the L/S driver circuit 2 generates a gate-off control signal (hereinafter, referred to as an off control signal) and outputs the signal to the gate of the FET₂.

Thus, since the FET₁ is turned on by an on control signal from the H/S driver circuit 1 and the FET₂ is turned off by an off control signal from the L/S driver circuit 2, a voltage in accordance with the input voltage V_(in) is supplied to the output terminal 4 through the smoothing circuit 13 including the inductor L₁.

When an output voltage Vout is supplied to the output terminal 4, a voltage Vor (=Vout×R₂/(R₁+R₂)) divided by the resistors R₁+R₂ connected between the output terminal 4 and the ground terminal 5 is input to the non-inverting input terminal of the comparator 10. When the divided voltage Vor is less than the reference voltage Vo input to the inverting input terminal, the output of the comparator 10 does not change.

Next, when an output voltage Vout which causes the divided voltage Vor to be greater than the reference voltage Vo is output from the output terminal 4, a high signal in accordance with a power-supply voltage corresponding to the input voltage Vin is output from the comparator 10. When a high signal is input from the comparator 10 to the on-time limiter circuit 11, the circuit 11 outputs the high signal to the L/S driver circuit 2 and the inverter 12 through the buffer 20. At this time, the NPN transistor Tr₁₁ in the on-time limiter circuit 11 is turned on by a base bias current supplied thereto, and the electric charge stored in the capacitor C₁₁ is discharged. Thus, the time-constant circuit returns to the initial state and the on-time limiter circuit 11 does not operate.

The inverter 12 outputs a low signal obtained by inverting the high signal input to the H/S driver circuit 1. When a low signal is input to the H/S driver circuit 1, the circuit 1 generates an off control signal and outputs the signal to the gate of the FET₁. On the other hand, when a high signal is input to the L/S driver circuit 2, the circuit 2 generates an on control signal and outputs the signal to the gate of the FET₂.

The FET₁ is turned off by an off-control signal input thereto, and the FET₂ is turned on by an on-control signal input thereto. Thus, the connection point between the drain of the FET₁ and the source of the FET₂ is made conductive to the ground terminal 5 through the passage between the drain and source of the on-state FET₂. In this manner, the output voltage Vout is reduced, such that the voltage Vds (substantially zero volt) between the drain and source of the on-state FET₂ is applied to the inductor L₁. At this time, a voltage obtained by subtracting the reverse voltage of the bootstrap diode D₀ and the voltage Vds between the drain and source of the on-state FET₂ from the input voltage Vin is applied to the bootstrap capacitor C₀ that is parallel-connected to the H/S driver circuit 1 to charge the capacitor C₀.

This operation continues until the divided voltage Vor becomes less than the reference voltage Vo, and, when the divided voltage Vor becomes less than the reference voltage Vo, as described above, the FET₁ is turned on and the FET₂ is turned off.

Then, an output voltage Vout having a desired voltage value (corresponding to a set voltage of the present invention) set by the reference voltage Vo is continuously output from the output terminal 4.

There are instances where an on-state FET₁ continues due to a transient load change. The operation of the ripple converter of the present preferred embodiment in this instance is described below.

When the output voltage Vout is less than a voltage set by the reference voltage Vo and the FET₁ in the on state continues, that is, when a divided voltage Vor in accordance with the output voltage Vout is less than the reference voltage Vo, the comparator 10 continues to output a low signal. When a low signal is output from the comparator 10, the NPN transistor Tr₁₁ in the off state in the on-time limiter circuit 11 continues. Thus, the input voltage Vin is applied to the capacitor C₁₁ through the resistor R₁₅ and the capacitor C₁₁ continues to be charged at a predetermined time constant. Then, when the voltage between both terminals of the capacitor C₁₁ reaches a certain value, a base bias current is supplied to the NPN transistor Tr₁₂ and the base obtains a high state to turn on the NPN transistor turns on. Then the base of the PNP transistor Tr₁₃ is pulled low and the PNP transistor Tr₁₃ is also turned on. Thus, a high signal in accordance with the input voltage Vin is input to the input portion of the buffer circuit 20 through the PNP transistor Tr₁₃ such that the PNP transistor Tr₁₃ is turned on. The buffer circuit 20 outputs the high signal to the inverter 12 and the L/S driver circuit, and the inverter 12 outputs a low signal inverted from the high signal to the H/S driver circuit 1.

When the H/S driver circuit 1 having the low signal input thereto outputs an off control signal, the FET₁ is turned off. On the other hand, when the L/S driver circuit 2 having a high signal input thereto outputs an on control signal, the FET₂ is turned on. At this time, the base of the NPN transistor Tr₁₁ is in a high state and the NPN transistor Tr₁₁ is turned on such that a high signal is output from the buffer circuit 20. As a result, the electric charge charged in the capacitor C₁₁ is discharged through the passage between the collector and emitter of the NPN transistor Tr₁₁. Accordingly, the NPN transistor Tr₁₂ is turned off and the PNP transistor Tr₁₃ is also turned off. As a result, the application of a high signal to the input portion of the buffer circuit 20 through the PNP transistor Tr₁₃ is stopped.

Even if such an operation is performed, since the divided voltage Vor in accordance with the output voltage Vout input to the non-inverting input terminal of the comparator 10 does not reach the reference voltage Vo, the comparator 10 continues to output a low signal. Then, since no high signal is applied to the input portion of the buffer 20 through the PNP transistor Tr₁₃, as described above, a high signal is input to the H/S driver circuit 1 and a low signal is input to the L/S driver circuit 2, and after that, the FET₁ is turned on and the FET₂ is turned off. Thus, the capacitance C₁₁ is charged at a predetermined time constant as described above. Then, the operation is repeated while the divided voltage Vor is less than the reference voltage Vo.

Here, the time constant τ_(b) of the bootstrap capacitor C₀ (affecting the discharge time) and the maximum pulse width τ_(p) of a low signal output from the on-time limiter circuit 11, which is mainly affected by the time constant R₁₅×C₁₁, are set so as to have the relationship of τ_(b)>τ_(p). When set in this manner, the time at which the bootstrap capacitor C₀ is discharged and the voltage for driving the H/S driver circuit 1 cannot be supplied and becomes less than the continuous on time of the FET₁ (the time where a low signal continues to be output from the on-time limiter circuit 11) to maintain secure control of the FET₁.

In particular, as described below, the ripple converter can be more securely and stably operated such that the continuous on time is set to be about three to about ten times as long as the operation cycle (continuous time of a pair of an on time and an off time) in the stable operation, for example.

First, since the voltage output from the output terminal Vout is obtained such that a voltage in accordance with the input voltage Vin is supplied through the inductor L₁ in the smoothing circuit 13 while the FETs are on-off controlled as described above, the voltage becomes a triangular wave which goes up and down around a set voltage that is set based on the reference voltage Vo. Then, the ripple of the output voltage depends on the ripple current of the inductor.

FIG. 3 shows waveforms of an output current of the load (current output from the output terminal), a ripple current of the inductor L₁, and a signal (switching control signal) output from the comparator.

Here, the amplitude of the ripple current of the inductor L₁ is dependent upon the inductance of the inductor L₁. For example, when the inductance is large, the amplitude AIL of the ripple current of the inductor is reduced. Thus, the amplitude of the output ripple current can be decreased to reduce the width of variation of the output voltage. On the other hand, in this manner, when the inductance of the inductor L₁ is increased, the speed of change of the coil current decreases in the transient state (at transient load changes) and the width of variation of the output voltage increases. In addition, the size of the inductor must be increased in order to increase the inductance. Accordingly, in order to trade off such advantages against disadvantages, normally the inductance is set so that the amplitude ΔIL of the ripple current of the inductor is about one third to about one half as large as the rated output current (current normally output from the output terminal 4).

Now, as shown in FIG. 3, when the load current is drastically changed from a low state to a high state, the FET₁ must be controlled so that the FET₁ may be turned on longer than normal operation. In this manner, the current flowing through the inductor L₁ increases to approximately the rated current, such that the FET₁ is continuously in the on state. Then, when the output current is close to the rated current, the FET₁ is on-off controlled and the rated current is maintained, as described above. At this time, the time in which the FET₁ is in the on state, that is, the time where the duty factor is 100% is about three times as long as the switching cycle in normal operation, that is, the repeating cycle of the on state, when the above rated output current is two to three times as large as the amplitude of the ripple current, although dependent on the duty factor in normal operation. Here, the bootstrap capacitor C₀ of the bootstrap circuit may be set so as to maintain the voltage for such a time. Accordingly, it is sufficient to set the maximum continuous time so as to be at least about three times the cycle of the switching control signal in normal operation, that is, in the stable state.

Next, when the input voltage Vin decreases to be substantially equal to the set voltage of the output voltage, for example, when the input voltage Vin gradually decreases, in the circuit structure described above, the input voltage Vin is continuously maintained to keep the FET₁ in the on state in order to maintain the output voltage. Then, in the above-described circuit structure, the FET₁ is temporarily off controlled at a predetermined timing in order to refresh the bootstrap capacitance C₀. There are cases in which the practical operation frequency of the circuit decreases and the ripple amplitude of the output voltage increase, so that such an operation is performed. This is dependent on characteristics of the smoothing circuit 13 including the inductor L1 and capacitor C₁.

FIG. 4 shows characteristics of the smoothing circuit 13, and, as shown in the drawing, in the DC-DC converter according to the present preferred embodiment, the switching frequency (repeating frequency of the switching control signal) f_(op) is set to be greater than the resonance frequency f_(r) of the smoothing circuit 13.

Typically, in designing an economical DC-DC converter, the operating frequency (switching frequency f_(op)) in normal operation is set to be about 100 times that of the resonance frequency f_(r). Furthermore, in order to suppress the ripple of the output voltage Vout within a range permitted for the DC-DC converter, the operating frequency (switching frequency f_(op)) typically must be at least about ten times that of the resonance frequency f_(r). Because of these restrictions, the continuous on time of the on-time limiter circuit 11 must be less than about ten times that of the switching cycle in normal operation.

Thus, by using the structure of the present preferred embodiment, even if the FET₁, which is a main switching element supplying switched Vin to the output terminal 4, is continuously in the on state, since an off state is temporarily inserted, the bootstrap capacitor is securely charged. In this manner, the present preferred embodiment solves the problem in the prior art. As a result, a ripple converter type DC-DC converter which stably operates is provided.

Furthermore, a ripple converter type DC-DC converter which more stably operates is provided such that the time limit in the on state is preferably set to be about three to about ten times as long as the switching control cycle in normal operation of the ripple converter.

While the present invention has been described with respect to preferred embodiments, it will be apparent to those skilled in the art that the disclosed invention may be modified in numerous ways and may assume many embodiments other than those specifically set out and described above. Accordingly, it is intended by the appended claims to cover all modifications of the invention which fall within the true spirit and scope of the invention. 

1. A DC-DC converter comprising: an Nch FET arranged to switch an input voltage on and off; a smoothing circuit arranged to smooth a voltage output from the Nch FET to output the smoothed voltage; and a comparator circuit arranged to compare a voltage output from the smoothing circuit with a set voltage to output a switching control signal for the on-off controlling; wherein when a continuous on time of the Nch FET is detected and the continuous on time reaches a predetermined time, a switching control signal for off controlling the Nch FET is output to the Nch FET.
 2. A DC-DC converter according to claim 1, wherein a time limit of the Nch FET in the on state is set to be about three to about ten times as long as a cycle of the switching control signal in normal operation.
 3. A DC-DC converter according to claim 1, wherein the smoothing circuit includes an inductor and a capacitor.
 4. A DC-DC converter according to claim 1, wherein the comparator circuit includes a comparator, a plurality of resistors and a reference power supply.
 5. A DC-DC converter according to claim 4, further comprising an on-time limiter circuit arranged to control the continuous on time of the Nch FET, wherein the on-time limiter includes a buffer circuit connected to a signal output terminal of said comparator via a resistor.
 6. A DC-DC converter according to claim 1, wherein said on-time limiter further comprises at least one NPN transistor and at least one PNP transistor.
 7. A DC-DC converter according to claim 1, further comprising a bootstrap circuit including a bootstrap diode and a bootstrap capacitor which are series-connected, wherein an anode of the bootstrap diode is connected to a drain of the Nch FET and an input terminal of DC-DC converter, and the bootstrap capacitor in connected to a source of the Nch FET.
 8. A DC-DC converter according to claim 1, further comprising an additional Nch FET connected between a connection point of Nch FET and the smoothing circuit and a ground terminal of the DC-DC converter.
 9. A DC-DC converter according to claim 1, further comprising an H/S driver circuit, wherein a gate of the Nch FET defines a control signal output terminal of the H/S driver circuit.
 10. A DC-DC converter according to claim 8, further comprising an L/S driver circuit, wherein a gate of the additional Nch FET defines a control signal output terminal of the L/S driver circuit.
 11. A DC-DC converter comprising: an Nch FET arranged to on-off control an input voltage; a smoothing circuit arranged to smooth a voltage output from the Nch FET to output the smoothed voltage; a comparator circuit arranged to compare a voltage output from the smoothing circuit with a set voltage to output a switching control signal for the on-off controlling; and an on-time limiter circuit; wherein when a continuous on time of the Nch FET is detected and the continuous on time reaches a predetermined time, the on-time limiter circuit outputs a switching control signal to the Nch FET for off controlling the Nch FET.
 12. A DC-DC converter according to claim 11, wherein a time limit of the Nch FET in the on state is set to be about three to about ten times as long as a cycle of the switching control signal in normal operation.
 13. A DC-DC converter according to claim 11, wherein the smoothing circuit includes an inductor and a capacitor.
 14. A DC-DC converter according to claim 11, wherein the comparator circuit includes a comparator, a plurality of resistors and a reference power supply.
 15. A DC-DC converter according to claim 14, wherein the on-time limiter includes a buffer circuit connected to a signal output terminal of said comparator via a resistor.
 16. A DC-DC converter according to claim 11, wherein said on-time limiter further comprises at least one NPN transistor and at least one PNP transistor.
 17. A DC-DC converter according to claim 11, further comprising a bootstrap circuit including a bootstrap diode and a bootstrap capacitor which are series-connected, wherein an anode of the bootstrap diode is connected to a drain of the Nch FET and an input terminal of DC-DC converter, and the bootstrap capacitor in connected to a source of the Nch FET.
 18. A DC-DC converter according to claim 11, further comprising an additional Nch FET connected between a connection point of Nch FET and the smoothing circuit and a ground terminal of the DC-DC converter.
 19. A DC-DC converter according to claim 11, further comprising an H/S driver circuit, wherein a gate of the Nch FET defines a control signal output terminal of the H/S driver circuit.
 20. A DC-DC converter according to claim 18, further comprising an L/S driver circuit, wherein a gate of the additional Nch FET defines a control signal output terminal of the L/S driver circuit. 